Conventionally, a technique firming a pipe body, for example, a prism pipe body by means of a bending process of a metal plate is disclosed in Japanese Patent Laid-Open No. Hei 11-290940.
According to the description disclosed in the gazette, a prism pipe body is formed of a metal plate of a rectangular shape by means of a press processing consisting of a plurality of processes.
That is, the processing method of the prism pipe body, comprises a first bending step of bending end parts of width direction of the metal plate along a length direction at the right angle to form a first processed piece having a bottom part connecting the opposite flange parts to each other, a second bending step of forming a recessed curve surface of a predetermined width along the length direction at the bottom part of the first processed piece, and at the same time, bending both ends of the recessed curve surface toward the inside at the almost right angle to form a second processed piece of a U shape having a pair of sidewall parts facing each other, and a re-striking step of pressing the pair of sidewall parts of the second processed piece toward each other to make the end surfaces of a pair of flange parts contact with each other.
According to the processing method of the prism pipe body, the recessed curve surface formed at the bottom part of the second processed piece plays a role of suppressing spring back force generated by pressing the pair of sidewall parts toward each other in forming the second processed piece, and therefore, the prism pipe body of which a cross-section is a square shape and in which both end surfaces of the flange parts are closed to each other can be manufactured by means of only the press forming, without welding both end surfaces of the flange parts.
However, in the conventional processing method of the prism pipe body, even if the recessed curve surface formed at the bottom part of the second processed piece plays a role of suppressing spring back force generated by pressing the pair of sidewall parts toward each other in forming the second processed piece, the spring back force which serves to unfold the pair of sidewall parts remains in the pair of sidewall parts, and thus, it was difficult to manufacture the prism pipe body in which both end surfaces of the flange parts are closed to each other, stably without deviation in mass-producing.
That is, when inspecting whether both end surfaces of the flange parts in the prism pipe body manufactured by the processing method are closed to each other, there are pipe bodies in which both end surfaces of the flange parts are closed to each other, while there are pipe bodies in which a gap is generated between both end surfaces of the flange parts by the spring back force. Also, the gaps are not constant.